Magnetic telegraphone



F. G. BUl-IRENDORF I Imus-21c 'unaemtaomi May 28, 1940.

Filed July 13, 1939 INVENTOR E G. BUHRENDORF BY ATTORNEY Patented May28, 1940 UNITED STATES MAGNETIC TELEGRAPHONE Frederick G. Buhrendorf,Hastings-on-Hudson, N. Y., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationJuly 13, 1939, Serial No.-28,141

5 Claims.

This-invention relates to magnetic telegraphones and particularly to themeans for mount ing the pole-pieces which cooperate with the record wireor tape.

A telegraphone pole-piece mounting should be of such a nature as topermit the pole-pieces to be spread easily to receive the tape and tofOllOW freely any lateral vibratory motion of the tape so as to keep thepressures of the pole faces on the tape substantially equal at all,times. While various mountings have been proposed heretofore to meetthese requirements, many of them involved pole-piece holders of such anature that the pole tips, under the force of friction with the tapesurface, vibrated or chattered slightly in a longitudinal sense, therebyintro! ducing noise and other distortion or became displaced one withrespectto the other thereby adversely afiecting the output and frequencyresponse of the telegraphone. Attempts to eliminate this chattering anddisplacement usually resulted in a sacrifice of the desired lateralflexibility. a

The object of this invention is a pole-piece mounting which eliminatesthis objectionable chatter without loss of lateral flexibility or otherdesirable features.

These results are obtained according to the invention by mounting eachpole-piece on oneend of a long flat spring extending some distancelaterally of the tape to the end of a main supporting spring disposed inthe plane of the tapeat some distance therefrom. The main spring ispreferably secured at the center to the frame of the unit with theindividual pole-piece springs secured to its free ends in pairs and atright angles to the main spring to form a substantially U-shaped springassembly. This provides a compact symmetrical structure which supportsboth the erasing and recording magnets and which may be readily replacedas a single unit.

These and other features of the invention will be more clearlyunderstood from the following detailed description and the accompanyingdrawing, in which Fig. 1 is a side view with the casing broken away toshow the mounting springs;

Fig. 2 is an end view; and

Fig. 3 is a top view of one embodiment of the invention.

' Referring to the drawing, the frame II is preferably of cast brass orother non-ferrous metal with four integral arms l2 for mounting the uniton the structure of the telegraphone by (oil. ire-100.2)

means of machine screws in. the holes l3. As shown more clearly in Fig.2, the upper part of the frame has two upwardly extending sidepieces M,M between which the tape I5 is moved by the reeling mechanism of themachine. The lower part of the frame comprises an L-shaped side-piece i6and the web member I I which terminates in a flat mounting surface l8(Fig. 3) in substantially the plane of the tape.

Secured at its center to this mounting surface is the main supportingspring l9 which is preferablyof phosphor-bronze about two inches long bythree-eighths inch wide and five mil-inch thick. Extending upwardly from.each free end of this spring are the pole-piece supporting springs 20,20 which are also of phosphor-bronze but'about one and one-half incheslong and somewhat narrower and thicker than the main spring 19. Thepole-piece springs are offset to form a. U between the arms of which thepolepieces 2|, 2|- are mounted with their pole faces engaging oppositesides of the tape [5.

The four pole-pieces 2| may, of course, be of any desired type but thoseshown are identical in shape and are preferably formed from sheetmolybdenum permalloy about 40 mil-inch. thick. The vertical dimension ofthe tip portions 22 will, of course, depend on the width of'the tape I5and they are'of sufficient length to mount a coil 23 without interferingwith the motion of the tape. The central portions of the polepieces arebent at right angles to form lugs 24 for attaching the pole-pieces tothe mounting springs 20 as shown in Fig. 1. Below the lugs 24 thepole-pieces have overlapping areas 25, 25 which are very large ascompared with the pole face areas. The pole-pieces 2 of each pair aremounted'in free, sliding contact with each other or with a verylsmallgap between them to eliminate .entirely any frictional force which wouldrestrict lateral motion. In either case, due to the large area ofoverlapping, there is a return path of very "low. reluctance .for theflux in the pole tips as explained in more detail in a copendingapplication of C. A. Lovell, A. H. Muller and D. E. Wooldridge SerialNo. 284,174, filed July 13, 1939.

When the pole tips are spread to receive the tape, the springs 20 aredeflected as shown in Fig. 2 thereby developing rest'oring forces whichhold the pole faces firmly in contact with the tape. If for any reasonthetape moves or vibrates laterally with respect to its normal line ofmovement, the spring l9, being thinner than the springs 20, will deflectin a torsional sense and the opposed pole-pieces and springs 20 willfollow the lateral motion of the tape as a unit. For the smallamplitudes of lateral motion which can occur the pressure of the polefaces on the tape will therefore be substantially unchanged.

While, as just explained, this mounting is quite free to move in a planenormal to the tape surface, the springs 19 and 20, being disposedsubstantially in the plane of the tape, present a very high stiffness toany forces tending to displace the pole-pieces longitudinally of thetape.

Assuming thatthe tape moves from left to right in Fig. 1, the left-handpole-pieces will be used for erasing and the right-hand pole-pieces forrecording and reproducing- The leads 26 from both of the coils 23 arebrought down to a pin-type plug 21 mounted in the base of the frame Iiso that the unit may be readily connected to the machine by merelyinserting the pins of the plug into a corresponding socket on themachine and securing the unit in place by means of set screws in thearms ii. For convenience of inspection or repair, the outer casing 28which protects the springs from mechanical injury is removably securedto the frame by set screws 29 and the tape cover is similarlyheld inplace by thumb screws 30.

It will be apparent thatthis pole-piece assembly is equally suitable forvarious types of magnetic recording systems and that although theoverlapping portions of the pole-pieces are in contact, or substantiallyso, the pole tips may be so shaped that the pole faces are aligned,longitudinally displaced or in some intermediate position as determinedby the type of recording desired. For best results the pole-pieces andsprings must be secured in their proper relative positions ratheraccurately but this is readily accomplished without tedious adjustmentsby assembling the parts in a suitable jig to determine their properalignment and then tightening the various machine screws whilemaintaining this alignment.

While the invention has been described for angles thereto insubstantially the plane of the a spring and a pair of companionpole-pieces secured in opposing relationship to the other ends of thespring members.

2. In a mounting-for telegraphone pole-pieces, a supporting structure, aU-shaped spring assembly comprising a thin flat spring member secured atits center to the structure and furcated arms each having two flatsprings normally disposed substantially parallel to the plane of thespring member, and a pair of companion pole-pieces mounted on thesprings of each of the furcated arms.

3. In a telegraphone, the combination with a moving tape record memberand two pole-pieces engaging opposite sides of the member, of a thinfiat spring mounted in the plane of the tape with one end free todeflect torsionally and in a direction normal to the plane of the tapeand two flat spring members secured at one end in the plane of the tapeto the free end of the spring extending toward the tape and holdingthe-pole-pieces in engagement with the tape.

4. A telegraphone pole-piece assembly com: prising a pair of pole-pieceshaving pole faces adapted to contact opposite sides of a tape recordmember and large wing extensions for forming a yoke between the polefaces, thin flat springs each secured at one end to one of thepole-pieces, and means of high stiffness in the plane of the pole facesand of low stiffness in the plane normal to the pole faces supportingthe springs in substantially the plane of the pole faces with the wingextensions in overlapping relation.

40 5. A pole-piece assembly for telegraphones comprising a mountingframe, a'thin, fiat, main supporting spring secured at its center to theframe, a pair of pole-piece supporting springs secured to each end ofthe main spring and extending substantially at right angles thereto, anda pair of companion pole-pieces mounted on each pair of pole-piecesprings and'having pole faces substantially in the plane of the mainsprina.

' FREDERICK G. BUHRENDORF.

